Thermosetting resins have been widely used as various electrical insulating materials, structural materials, adhesives and the like using casting, impregnating, laminating, and molding techniques. In recent years, severer requirements have been imposed on the materials used in these applications. Inter alia, heat resistance is an important requirement.
Thermosetting polyimide resins generally used as thermosetting resin are quite resistant against heat, but poor in processing since they must be heated at high temperatures for a long period of time upon working. In turn, epoxy resins modified for heat resistance improvements are easy to process, but insufficient in mechanical properties at high temperatures, electrical properties, long-term thermal degradation resistance, and heavy-duty heat resistance.
Several replacements for these thermosetting resins are known, for example, a thermosetting mixture containing a polyimide and an alkenylphenol or alkenylphenol ether (see Japanese Patent Application Kokai No. 994/1977), and a heat resistant resin composition containing a maleimide compound, a poly(allylated phenol) compound and an epoxy resin (see Japanese Patent Application Kokai No. 184099/1983). The poly(allylated phenol) compounds used in these compositions have a nuclearly substituting allyl group and a hydroxyl or ether group attached to a common aromatic ring at ortho positions, because they result from Claisen rearrangement of a poly(allyl ether) compound or have the structure that a phenolic hydroxyl group is generated through Claisen rearrangement during thermosetting. Therefore, they tend to remain unreacted, particularly in resin compositions of the novolak type, leaving problems regarding setting properties and thermal degradation resistance at high temperatures.